Fuel cells can be used for the environmentally friendly generation of electricity. This is because a process which substantially represents a reversal of electrolysis takes place in a fuel cell. For this purpose, in a fuel cell, a fuel which includes hydrogen is fed to an anode and an auxiliary substance which includes oxygen is fed to a cathode. The anode and cathode are electrically separated from one another by an electrolyte layer; although the electrolyte layer does allow ion exchange between the fuel and the oxygen, it otherwise ensures gas-type separation of fuel and auxiliary material.
On account of the ion exchange, hydrogen contained in the fuel can react with the oxygen to form water, during which process electrons accumulate at the fuel-side electrode, i.e. the anode, and electrons are depleted at the electrode on the auxiliary substance side, i.e. the cathode. Therefore, when the fuel cell is operating, a potential difference or voltage is built up between the anode and cathode. The electrolyte layer, which in the case of a high-temperature fuel cell may be designed as a solid ceramic electrolyte or in the case of a low-temperature fuel cell may be designed as a polymer membrane, therefore has the function of separating the reactants from one other, of transferring the charge in the form of ions and of preventing an electron short circuit.
On account of the electrochemical potentials of the substances which are usually used, in a fuel cell of this type, under normal operating conditions, an electrode voltage of approximately 0.6 to 1.0 V can be built up and maintained during operation. For technical applications, in which a significantly higher overall voltage may be required depending on the intended use or the planned load. Therefore, it is usual for a plurality of fuel cells to be connected electrically in series, in the form of a fuel cell stack, in such a way that the sum of the electrode voltages which are in each case supplied by the fuel cells corresponds to or exceeds the required total voltage.
Each of the fuel cells which are combined to form a fuel cell stack of this type is assigned, in the region of its electrode, a volume region to which the media required in each case, such as for example the fuel or the auxiliary material, can be fed. This volume region may, for example, be delimited by boundary surfaces which are counted as part of the fuel cell itself, the boundary surfaces between two adjacent fuel cells, in order to form a closed volume region, being sealed off from the outside by means of a seal arranged between them. Depending on the total voltage required, the number of fuel cells in a fuel cell stack of this type may, for example, be 50 or more.
To produce the required seal between adjacent fuel cells in terms of the supply and discharge of the media, such as fuel and auxiliary substance, it may be necessary to subject the fuel cell stack to a certain clamping or pressing in its longitudinal direction. This corresponds to a long-term compressive load acting on the fuel cell stack in its longitudinal direction. This ensures that, firstly each fuel cell remains in mechanical contact with the fuel cells adjoining it, and, secondly, particularly when seals made from elastic material are used between the fuel cells, the required sealing action is indeed achieved as a result of the pressure applied in the longitudinal direction.
However, with elongate structures, such as bars, towers or struts, according to the Euler or Tetmajer buckling conditions, such a compressive load may result in a tendency to buckle. In this context, the term buckling is to be understood in particular as meaning yielding of a central region of the elongate structure in a direction perpendicular to the longitudinal axis. This tendency to buckle is dependent to a considerable extent on the length of the structure in question.
Buckling of this nature which occurs in a fuel cell stack, as a result of some of the fuel cells being moved out of their desired position, would, however, have a highly adverse effect on or even completely negate the ability of the fuel cell stack to function. The number of fuel cells which can be connected to one another to form a fuel cell stack is therefore only limited, depending on the sealing system which is provided for the connection of adjacent fuel cells and the resulting clamping force required in the longitudinal direction of the fuel cell stack.
On the other hand, however, particularly when a fuel cell system is designed for applications with relatively high design voltages, it is possible to provide for a relatively high number of fuel cells, for example 70 or more, to be connected up. The possibility of combining any desired number of fuel cells to form a fuel cell stack therefore represents an important contribution to the flexibility available in designing a fuel cell module. In particular for flexibility reasons, it may be desirable for the fuel cells to be combined to form a fuel cell stack in a readily portable fuel cell module.